Hey Ruth,
First-of-all, Twitter is written IN Python.
Python is still going strong, no doubt about it.
It is one of the most used languages in AI, Machine Learning and Deep learning circles, primarily because, the syntax is concise, the interpreter footprint in low. And, since it has been around for so long, the amount of libraries it has is just amazing.
I have built multiple projects in Python, including, VoIP traffic monitoring and fraud detection systems, IoT projects for some home automation, AI based projects to analyse datasets and predict user behavior based on cognitive models.

Django - Yes, for a full featured MVC portal, for an Enterprise Application or something. But, consider Flask too, you mentioned React is getting popular, but, that's just the frontend. Flask gives you the ability to do microservices in a jiffy and does not carry the overweight modules as in the case of Django.